The use of crystalline aluminosilicate zeolites for catalyzing the alkylation of benzene with olefins is now well known in the art. It is customary in such processes to utilize a fairly large mole-excess of benzene in order to minimize polymerization of the olefin, as well as to promote the formation of monoalkyl benzene and suppress the formation of polyalkyl benzenes. The polyalkyl benzenes are in many cases subjected to transalkylation with benzene to produce additional monoalkylbenzene. Such transalkylation can be effected either in a separate transalkylation zone, or by recycling the polyalkylbenzenes to the alkylation zone.
A major problem associated with such zeolite-catalyzed alkylations has been that of catalyst deactivation rates, which usually limit run lengths to no more than a few weeks before the catalyst must be regenerated. It has been fairly well established that the mechanism of deactivation involves polymerization of the olefin, followed by hydrogen-transfer and cyclization reactions to form large aromatic molecules which cannot diffuse out of the crystal micropores of the zeolite in which the active sites are located. (Venuto et al, J. Catalysis 5, 484-493, 1966; I and EC Product Research and Development, 6, 190-192, Sept. 1967).
In alkylating benzene with ethylene using a catalyst of this invention (in the form of 1/16" cylindrical extrudates) and with no recycle of polyalkylbenzenes to the alkylation zone, the catalyst deactivation rate was such as to indicate a maximum run length of about 93 days under a specific set of operating conditions. Under the same conditions, but with recycle of the diethyl- and triethylbenzene product fraction to the alkylation zone, the deactivation rate was such as to indicate a maximum run length of only about 55 days. However, most unexpectedly, in two separate run periods in which only the diethylbenzene product fraction was recycled, there was essentially no detectable catalyst deactivation, indicating a run length of at least one year or more. Apparently, the presence of diethylbenzenes at the inception of alkylation, i.e., at the inlet end of the reactor where the concentration of ethylene is still high, in some manner stabilizes the system, an effect which is destroyed by the presence of triethylbenzenes, or possibly some polymer which cannot be readily separated from triethylbenzenes by distillation.
According to the present invention therefore, diethylbenzenes, but essentially no triethylbenzenes, are recycled to the alkylation zone to achieve a stable long-lived alkylation cycle. However, to achieve this objective it is not necessary, nor is it desirable, to recycle all of the diethylbenzene fraction. Maximum transalkylation efficiency generally requires higher temperatures than are optimum for alkylation, and hence a separate transalkylation zone is provided to which benzene, a large proportion of the diethylbenzenes, and all of the triethylbenzenes and higher alkylated products are fed for conversion to ethylbenzene. Another advantage is not recycling all of the diethylbenzene fraction is that the distillation load is reduced since it is not necessary to make a sharp separation between diethylbenzenes and triethylbenzenes.